1. Field of the Invention
The present invention relates to an imaging sheet for achieving color balance in a full color imaging system. More particularly, the present method corrects color balance for a full color imaging system including three sets of pressure rupturable photosensitive microcapsules by incorporating a corrective dye into the system.
2. Description of the Prior Art
When producing full color images, it is commonly known in the art to utilize three sets of photosensitive microcapsules, each containing a different color-forming agent and a photocurable composition which is sensitive to a particular wavelength of light. In a typical system, the microcapsules contain cyan, magenta, and yellow color-forming agents and photocurable compositions which are respectively sensitive to wavelengths in the red, green, and blue regions of light. Such a system is described and disclosed U.K. Patent No. 2,113,860, the contents of which are herein incorporated by reference.
In a known full color system, each microcapsule contains a photohardenable composition including a radiation curable composition which undergoes an increase in viscosity upon exposure to actinic radiation and a photoinitiator. Each photoinitiator is particularly sensitive to a given band of actinic radiation.
Depending on the radiation source used, the photoinitiators selected determine, in a large part, the film speed of the respective microcapsules. If the photoinitiators react with light with different efficiencies, the microcapsules will have different film speeds. As a consequence, for a predetermined amount of radiation, the microcapsules will release different amounts of color-forming agents and the colors produced by the three sets of microcapsules will not be balanced. In addition, color imbalance can occur as a result of the different spectral energy characteristics exhibited by the light source utilized.
Attempts have been made to correct color imbalance caused by either the photoinitiators contained in the microcapsules or the radiation source. A first method involves using a trial and error technique to adjust the relative proportion of the amounts of microcapsules to be coated onto a substrate. In theory, the substrate could contain one-third cyan color-forming microcapsules, one-third magenta color-forming microcapsules and one-third yellow color-forming microcapsules. However, if the film speed of a particular color-forming microcapsule is faster than the film speed of the other two color-forming microcapsules, higher amounts of the faster speed microcapsules or lower amounts of the slower speed microcapsules could be coated onto a substrate to create color balance.
In another method, the light emitted by the radiation source may be altered to provide color balance. This typically involves the use of color or ultraviolet filters which alter the spectral energy characteristics of the radiation source.
Although both of the above described methods have been utilized to control color balance, it would be desirable to develop an imaging sheet for achieving color balance in a full color imaging system without altering either the amounts of microcapsules coated onto a substrate or the radiation source utilized.